%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-1-37 on page I-284 in "Defining the style of headed lists".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{135.0pt}
% small headings for the example
  \makeatletter
  \renewcommand\section{\@startsection{section}{1}{\z@}%
    {-1.25ex \@plus-1ex \@minus-.2ex}%
    {.5ex}%
    {\normalfont\large\bfseries}}
  \makeatother

  \usepackage{microtype}  % improve the line breaking in short measure


%StartShownPreambleCommands
\usepackage{amsthm}
\theoremstyle{plain}      \newtheorem{thm}{Theorem}[section]
\theoremstyle{definition} \newtheorem{defn}[thm]{Definition}
\theoremstyle{remark}     \newtheorem*{rem}{Remark}
%StopShownPreambleCommands

\begin{document}
\section{Theorem-Like}
\begin{defn}
 A typographical challenge is a problem that cannot be
 solved with the help of \emph{The \LaTeX{} Companion}.
\end{defn}
\begin{thm}[Main]
     There are no typographical challenges.\end{thm}
\begin{rem}The proof is left to the reader.\end{rem}
\end{document}
